Doubly balanced mixers are widely used in telecommunications equipment (receivers, transmitters), radar, television, electronic instrumentation, etc . . . , whenever it is required to convert an incident signal of given frequency into an output signal of a different frequency, by additive or subtractive mixing with a high level auxiliary signal of suitable frequency.
The invention relates more particularly to doubly balanced mixers of the type comprising a ring modulator with four identical rectifying legs each comprising at least one diode.
A first transformer comprises a primary winding receiving the incidence signal (commonly called "RF port"), and a secondary winding with central tapping; a second transformer, similar to the first one, comprises a primary winding receiving the auxiliary signal (commonly called "LO port") and a secondary winding with central tapping. The ring modulator has its diagonals connected respectively to the terminals of the secondary windings of the two transformers and an "output port", commonly called "IF port", is connected to the central tappings of said secondary windings.
The known theory of such modulators shows that the IF output signal has two main frequency components given by the formula F.sub.IF =F.sub.LO .+-.F.sub.RF, the desired component being selected for example by filtering.
In a first approximation, the different modulations which affect the RF signal (amplitude, phase, frequency and combinations thereof) also affect the IF signal.
Since the level of the auxiliary LO signal is assumed sufficiently high to cause alternate conduction of the two pairs of rectifiers, assumed perfect, which form the modulator ring and since the transformers are assumed to be perfect and of a ratio 1/2 (each half of the secondary comprising as many turns as the primary), theory shows that the amplitude of each component of the IF signal is equal to that of the RF signal multiplied by 2/.pi., i.e. an attenuation of 4 dB.
In practice, the balanced mixers of the prior art are tainted with different defects due to the losses in the transformers (additional attenuation) and especially to the imperfection of conventional high-speed diodes which form the legs of the ring modulator, which causes an attenuation increasing at the same time as the level of the RF signal (saturation), and the appearance of undesirable parasite signals corresponding to a distorsion and to intermodulation products.